fix: store create operation as object in this.#ops

[nikolockenvitz]

fixes #49 @csuwildcat

As explained there, the create operation was incorrectly stored as a Promise in the operation queue, leading to errors when trying to generate a request for a subsequent operation.
(The create operation was always stored as a Promise, but previously all the operations were awaited with Promise.all() in generateOperation, so that previous always was a pointer to the plain object. With the recent changes, this.#ops was accessed directly and no longer awaited, leading to the described problem. Storing the create operation as a promise is not necessarily incorrect - I just don't see a reason do it it like this while all other operations are stored as plain objects).

Consequently, it would not be necessary anymore for getAllOperations() to be async / use Promise.all(). Same for getOperation().

Is it planned to add tests to this library and/or to migrate to TypeScript? This would have caught the error.